NOT FOR the first time, the debate about Syria in Washington is being overtaken by developments on the ground. For months the Obama administration has been resisting proposals that it support the creation of safe zones for the Syrian opposition or supply the rebels with arms. Yet now reporting from inside Syria, including by Western journalists, shows that big pieces of territory have already been taken over by opposition groups and that significant quantities of weapons are flowing to their fighters.

While the regime of Bashar al-Assad continues to control large cities and maintains outposts elsewhere, the opposition dominates much of the countryside, according to a new report by Joseph Holliday of the Institute for the Study of War. The largest of the liberated areas, he says, extends from Syria’s northwestern border with Turkey to the main north-south highway between Damascus and Aleppo, and south to the outskirts of the central city of Hama. Government forces that have tried to penetrate the area have been met with roadside bombs, volleys of rocket-propelled grenades and heavy machine guns mounted on trucks.

The growing rebel strength is welcome news, because it suggests that the Assad regime will not be able to restore its control over the country by force. But it raises new problems for those who seek to prevent Syria from tumbling into chaos or a sectarian war that spreads to its neighbors. Though able to hold onto rural territory, the rebels appear far from being able to challenge the regime’s control over Damascus or other cities — which suggests that the war could drag on for months or years. The Assad army, meanwhile, is resorting to more extreme measures of force: It recently began deploying helicopter gunships.

Mr. Holliday counted some 300 armed rebel groups with an estimated 40,000 fighters. Though some are organized under provincial commands, it’s not clear that any are able to impose order on the ground they control, even as government structures disappear. In effect, the Syrian state is collapsing even as the Assad regime and the military and militia units loyal to it fight on. So far there is nothing to replace it.

Following the entirely predictable failure of a U.N. peace mission, the Obama administration is working on an initiative to obtain international agreement on the terms of an eventual Syrian transition, including Mr. Assad’s departure from Syria and free elections. If it can be reached, such an accord could eventually prove useful, but it doesn’t address the rapidly changing situation in Syria. What’s needed is an aggressive effort to shape and support the emerging rebel organizations, aimed at helping them to consolidate control over territory, communicate with each other and establish governing structures.

The Obama administration should also be asking itself and its allies whether it can speed the rebels’ military development; the longer the war lasts, the greater the chance that extremists will win. And it should consider what it will do if, as can be expected, the Assad regime mounts major new offensives against the rebel enclaves, using aircraft and unleashing the militias that have been committing massacres. To remain passive in such an instance should not be an option.